Contrib Mineral Petrol (2004) 147: 363–383 DOI 10.1007/s00410-004-0565-3 ORIGINAL PAPER Paolo Nimis Æ Svetlana G. Tesalina Æ Paolo Omenetto Paola Tartarotti Æ Catherine Lerouge Phyllosilicate minerals in the hydrothermal mafic–ultramafic-hosted massive-sulfide deposit of Ivanovka (southern Urals): comparison with modern ocean seafloor analogues Received: 15 August 2003 / Accepted: 9 February 2004 / Published online: 17 March 2004 Ó Springer-Verlag 2004 Abstract We have studied textural relationships and required a high contribution of Mg-rich seawater to the compositions of phyllosilicate minerals in the mafic– hydrothermal fluid, which could be achieved in a highly ultramafic-hosted massive-sulfide deposit of Ivanovka permeable, breccia-dominated seafloor. More evolved (Main Uralian Fault Zone, southern Urals). The main hydrothermal fluids produced addition of silica, car- hydrothermal phyllosilicate minerals are Mg-rich chlo- bonates and further sulfides, and led to local develop- rite, variably ferroan talc, (Mg, Si)-rich and (Ca, Na, ment of saponite after chlorite and widespread K)-poor saponite (stevensite), and serpentine. These replacement of serpentine by talc. The Ivanovka deposit minerals occur both as alteration products after mafic shows many similarities with active and fossil hydro- volcanics and ultramafic protoliths and, except serpen- thermal sites on some modern oceanic spreading centers tine, as hydrothermal vein and seafloor mound-like characterized by highly permeable upflow zones. How- precipitates associated with variable amounts of (Ca, ever, given the arc signature of the ore host rocks, the Mg, Fe)-carbonates, quartz and Fe and Cu (Co, Ni) most probable setting for the observed alteration–min- sulfides. Brecciated mafic lithologies underwent perva- eralization patterns is in an early-arc or forearc seafloor– sive chloritization, while interlayered gabbro sills subseafloor environment, characterized by the presence underwent partial alteration to chlorite + illite ± of abundant mafic–ultramafic breccias of tectonic and/ actinolite ± saponite ± talc-bearing assemblages and or sedimentary origin. later localized deeper alteration to chlorite ± saponite. Ultramafic and mixed ultramafic–mafic breccias were altered to talc-rich rocks with variable amounts of chlorite, carbonate and quartz. Chloritization, locally Introduction accompanied by formation of disseminated sulfides, Phyllosilicate minerals are a major constituent of low- grade metamorphic and hydrothermally altered mafic– Editorial responsibility: J. Hoefs ultramafic rocks. Their association with massive sulfide deposits is well known both in modern ocean seafloor P. Nimis (&) Æ S. G. Tesalina Æ P. Omenetto hydrothermal sites and in their ancient counterparts, Dipartimento di Mineralogia e Petrologia, and analogies between modern and ancient examples Universita` degli Studi di Padova, C.so Garibaldi 37, 35137 Padova, Italy have been pointed out by several workers (e.g. Costa E-mail: [email protected] et al. 1983; Kranidiotis and MacLean 1987; Evans and Tel.: +39-049-8272022 Guggenheim 1988; Zierenberg et al. 1995). The nature Fax: +39-049-8272010 and compositions of the phyllosilicate minerals can be P. Nimis Æ P. Omenetto used to discriminate between various metamorphic Kroseven Services S.A.S., Padova, Italy and hydrothermal environments (e.g. Shikazono and S. G. Tesalina Kawahata 1987; Laird 1988), to gain an insight into Institute of Mineralogy, Uralian Division, hydrothermal processes at or near the seafloor (Aggar- Russian Academy of Sciences, 456301 Miass, Russia wal and Nesbitt 1984; Evans and Guggenheim 1988), as P. Tartarotti well as to estimate physical and chemical conditions Dipartimento di Scienze della Terra, extant during rock–fluid interaction and hydrothermal Universita` di Milano, Milano, Italy precipitation (e.g. Cathelineau and Nieva 1985; Cathe- C. Lerouge lineau 1988; Zierenberg and Shanks 1988). Formation of BRGM, B.P. 6009, 45060 Orle´ans Cedex 2, France phyllosilicate minerals during hydrothermal alteration is 364 often associated with significant removal or uptake of marks the main suture that can be traced continuously metals and silica. These processes contribute to the along the Uralide Orogen (e.g. Puchkov 1997) (Fig. 1). compositional evolution of the hydrothermal fluids and The MUFZ is interpreted as the major damage zone that produce geochemical and mineralogical zoning within developed between an accretionary wedge to the west hydrothermal recharge and upflow zones (Alt 1995). and a forearc basement to the east during Late Devo- The results presented herewith form part of a com- nian arc–continent collision (Brown and Spadea 1999). prehensive study of mineralizations and ore–host rock It comprises a me´lange of dismembered mafic–ultra- relationships in mafic–ultramafic-hosted, massive sulfide mafic sheets made of serpentinites, gabbros, basalts, deposits in southern Urals. In this paper we describe dolerites, mingled with Mid-Ordovician to Mid-Devo- phyllosilicate mineralogy in a little known deposit from nian sedimentary rocks (e.g. Maslov et al. 1993; Serav- the Main Uralian Fault Zone, located near the village of kin et al. 2001). The sedimentary units include Ivanovka (Fig. 1). We will show that this deposit con- olistostromes which carry olistolites of volcanic and stitutes an example of fossil hydrothermal body in which intrusive igneous rocks and, sometimes, seafloor the original seafloor–subseafloor hydrothermal mineral hydrothermal mounds of carbonatic and carbonatic- assemblages and textures have been extraordinarily well siliceous composition. preserved. The compositions and paragenetic relations In the southern Urals, important massive sulfide of the Ivanovka phyllosilicates will be discussed through deposits belonging to the Cyprus, Besshi, Baimak comparisons with those reported for hydrothermal (Kuroko), and Uralian types are associated with Mid- deposits on modern seafloor settings. Palaeozoic ophiolitic and island-arc volcanic sequences (Prokin and Buslaev 1999; Herrington et al. 2002). Mafic–ultramafic-hosted massive sulfide occurrences Geological outline along the southern MUFZ (i.e. Ivanovka, Dergamish, and Ishkinino deposits) are represented by small-sized, The Urals are a linear orogenic belt that resulted from at present subeconomic, Cu (Co, Au) deposits (Zaykov the Late-Palaeozoic collision of the East European et al. 2000; Herrington et al. 2002). Sulfide mineraliza- Platform with a Siberian–Kazakhian plate assemblage tion in these deposits is traditionally thought to occur and interposed oceanic and island-arc terranes. The near or at the contact between variously metasomatized Main Uralian Fault Zone (hereafter MUFZ), a 2 to 10- serpentinitic and basaltic units (e.g. Ismagilov 1962; km-wide (locally up to 25 km), east dipping fault system, Buchkovskiy 1970; Zakharov and Zakharova 1975; Zaykov et al. 2000). This idea may not be completely correct. Based on petrographic and geochemical analy- ses (see Mineral textures and hydrothermal parageneses), we believe that many ore-bearing rocks that have been described in the literature as ‘‘metasomatized serpenti- nite’’ (or ‘‘aposerpentinity’’ in the Russian lit.) actually include a me´lange of hydrothermally altered mafic and ultramafic lithologies of both crustal and mantle derivation. The Ivanovka deposit At Ivanovka, basalts of post-Silurian, possibly Early- Mid Devonian age (Seravkin et al. 2001) rest on an extensively metasomatized sequence of mafic–ultramafic rock breccias cross-cut by several pre-metasomatic gabbroic sills, decimetric to metric in thickness (Figs. 2 and 3). The mixed mafic–ultramafic rock sequence hosts the sulfide mineralization and lies on a serpentinite unit. Metamorphism, probably related at least in part to oceanic hydrothermal alteration, is mostly represented by serpentinization of former harzburgites and dunites, and by albite + chlorite + epidote + quartz greens- chist-facies metamorphism of mafic rocks. The latter are well exposed on an escarpment along the Yangyz River, a few hundred meters north-east of the ore deposit. The sulfide mineralization comprises massive, dis- Fig. 1 Geological sketch-map of the southern Urals, with location seminated and stockwork ores which consist of domi- of the Ivanovka ore field nant tabular, lamellar and granular pyrrhotite, with or 365 portions of talc, (Ca, Mg, Fe)-carbonates, chlorite, saponite, and quartz (Zakharov and Zakharova 1975; and in-house unpublished data). The sulfide-rich levels are mainly (75%) restricted to chlorite-rich rocks after mafic protoliths, where also higher Cu (Co, As, Au) occur (ca. 1% Cu). Relatively high Ni contents (0.1– 0.2%), with low Cu (Co, As, Au), are mostly found in talc-rich, ultramafic-hosted mineralizations. Significant Au contents (up to 5.4 ppm) are found near the top of the ore-bearing sequence within pyrite + chalcopyrite + pyrrhotite-bearing assemblages. The Ivanovka deposit differs from sulfide deposits within the Uralian back-arc and island-arc terranes by its extremely poor Zn grades (<to <<0.1%) and Co– As–Ni–Bi (Au) specialization (Tesalina et al. 2001). The aggregates of lamellar pyrrhotite that occur at the top of the deposits strictly resemble those found in some modern seafloor hydrothermal mounds (Peter and Scott 1988; Goodfellow and Franklin 1993; Turner et al. 1993; Zierenberg et al. 1993), suggesting formation near or at the seafloor. However, the presence of stockwork min- eralizations, the widespread association of sulfides with alteration minerals, the presence of inclusions